Check your local bottle recycling depot. I got a whole bunch of Grolsch bottles for next to nothing ($60 for 400). I am told that many depots can't handle swing-top bottles, and they end up getting trashed. I called my local depot and they held all them for me and gave me a call when they had a bunch.

Otherwise, EZ-cap is the biggest commercial distributor. Call them to see if there is a reseller near you.

There's another recent post in the bottling section about this.
Options:
1) Buy/drink grolsch, christoffel, fischer, or a variety of german beers that come in swingtops. Buy full cases to save $$ if you can. Grolsch is widest available and at ~$2.20 each, it's like buying an empty from an HBS with free beer inside.
2) Craiglist
3) Ask at bars, liquor stores, distributors, recycling centers, etc. In states like Michigan liquor stores have to take them back and return them to distributors. I buy them for $0.10 apiece (the cost of the deposit).
4) Worst case - buy empties.